The invention relates to an image forming apparatus of, for example, the electrophotography type which may be used in a copier, a printer, a facsimile apparatus, or the like.
Conventionally, image forming apparatuses employing the electrophotography technique are popularly used. In such an image forming apparatus, usually, a photosensitive body (for example, a photosensitive drum) which is formed into a drum or a belt is uniformly charged, and light carrying image information which is recorded on, for example, an original impinges on the photosensitive drum so that an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image is developed by a developer (toner), and the developed image is transferred onto a sheet directly or via a transfer drum or the like, thereby obtaining a print (copy) image. In such an image forming apparatus of the electrophotography type, a corona discharger for charging or destaticizing the photosensitive drum is widely used.
When a corona discharger is operated, discharge products such as ozone and nitrogen oxides are produced. Such discharge products are harmful to the apparatus and also to the environment. Usually, a forced air flow caused by an exhaust blower or the like is emitted to the outside of the apparatus through a filter so that discharge products are decomposed or adsorbed by the filter, or the forced air flow is emitted to the outside of the apparatus through an exhaust duct having an inner wall to which a substance which decomposes or adsorbs discharge products is applied, whereby the discharge products are decomposed or adsorbed by the applied layer (for example, see Japanese patent publication (Kokai) No. SHO.58-190968, Japanese utility model publication (Kokai) Nos. HEI.2-09052, and HEI.2-111142, and Japanese patent publication (Kokai) No. HEI.4-128774).
Because of the internal structure of the apparatus, however, a stagnant portion may be formed which is, for example, in the vicinity of the corona discharger and into which discharge products easily enter, but which is not affected by the air flow. In the case where the opening of such a stagnant portion or that of an opening portion which communicates with the stagnant portion and is formed as an air flow path is directed downward or obliquely downward and opposes the photosensitive drum, there may arise a situation in which discharge products flow out through the opening, for example, after the operation of the apparatus is ended, and then contact the photosensitive drum because such discharge products contain components heavier than air.
When color superimposition such as color copying is to be conducted, air is exhausted from the portion above a scorotron so that toner is prevented from adhering to a grid. In this case, particularly, discharge products easily move to the surface of a photosensitive body.
A photosensitive drum which is used in an image forming apparatus of the electrophotography type has a tendency to very sensitively react due to the environment so as to deteriorate its properties. Since such a photosensitive drum must always start operation under the same initial conditions, it stops in a fixed initial state in which the same portion is always directed in the same direction.
During a stop period, therefore, discharge products are always in contact with the same portion of the photosensitive drum, with the result that only that portion is locally deteriorated. This produces a problem in that a distinct change in density and contrast appears in a part of an image printed (copied) on a sheet.
In order to prevent such a problem from occurring, it may be contemplated to design the apparatus so as not to form a stagnant portion. In many cases, however, it is very difficult to adopt such a design because of restriction due to the internal structure of the apparatus.